Conventionally in industrial radiographic inspection, a beam of high-energy X-ray radiation is transmitted through a test object to be inspected, and an image of the test object is formed by interaction of the transmitted radiation with silver halide in a photographic film. A flaw, defect or structural inhomogeneity in the test object is detected by examining (or "reading") the X-ray image formed on the photographic film. The reading of X-ray images on photographic film, however, is inherently a subjective procedure, and is subject to idiosyncratic variations depending upon the skill and knowledge of the individual human agent who does the reading. Furthermore, the requirement that the photographic film be chemically developed in order to form the image precludes real-time inspection of the test object.
Image subtraction procedures have not generally been used in industrial radiographic inspection. In clinical radiology, on the other hand, image subtraction procedures have been used since the 1930's for suppressing the appearance of bony structures overlying anatomical regions of interest. With image subtraction techniques used in clinical radiology, two images of the region of interest are formed in sequence. The images may be formed by chemical means (i.e., on photograhic film) or by electronic means. The first image is representative of the region of interest in its natural state, and the second image is formed after a radiopaque medium has been injected into the region. If the position of the region of interest relative to the X-ray source and relative to the image-forming means (i.e., photograhic film or electronic image-forming device) remains the same for the first and second images, subtraction of one of the images from the other to form a subtractive image leaves only the radiopaque medium to be seen. Thus, by studying the subtractive image, one can view an outline of the region of interest (as defined by the radiopaque medium) in sharp contrast with surrounding tissue.
An image subtraction procedure described in an article entitled "A Multiple Image Subtraction Technique for Enhancing Low Contrast, Periodic Objects" by C. A. Mistretta et al., Investigative Radiology, Vol. 8, No. 1, (1973), pp. 43-49, uses electronic image forming means and eliminates the need for injecting a radiopaque medium into the region of interest. In the procedure described by Mistretta et al., X-ray transmission differences associated with periodic mass changes that occur in certain tissues are used to form subtractive images in which background anatomical structures are suppressed. More particularly, the procedure described by Mistretta et al. takes advantage of the time-dependent variation in blood mass distribution between systolic and diastolic states of pulmonary arteries to generate arteriograms by an electronic image subtraction technique.
In industrial radiography, manufactured articles, which do not undergo time-dependent internal structural changes, have not heretofore been considered amenable to radiographic inspection by image subtraction procedures.